1. Field of the Invention
The present invention relates generally to RF transmission line components, and particularly to microwave ferrite circulator/isolator devices.
2. Technical Background
A ferrite circulator/isolator is a passive multi-port microwave device that is typically employed in RF transmission line applications such as radar, cell phone applications, etc. The ferrite circulator/isolator device is typically used to provide a low loss transmission path for RF energy in one direction and substantially prevent any transmission of energy in the reverse direction. In a typical communications device, an RF signal may be modulated, amplified and directed to an antenna for transmission over a communication channel. If a reflected RF signal or some other RF signal is permitted to propagate in the reverse direction, an unprotected signal source may be significantly damaged. The ferrite circulator/isolator device is configured to attenuate such RF transmissions to thereby prevent such damage from occurring.
A typical ferrite circulator includes three ports, and is generally referred to as a Y-junction circulator. In operation, when an RF signal is directed into a first port, the RF signal will be accessible via the second port in sequence, i.e., the port immediately adjacent the input port. The RF signal will be substantially attenuated and will not be available at the third port in the sequence, that is, the port immediately adjacent to the second port on the other side of the first input port. On the other hand, if an RF signal is directed into the second port, it will be available as an RF output signal at the third port, but will not be available at the first port. Finally, if an RF signal is introduced at the third port, it will be available as an RF output at the first port, but not at the second port. A circulator, therefore, propagates RF power from one adjacent port to the next in a sequential, circular fashion. The RF signal circulation may be right-handed (RH) or left-handed (LH).
The circulation action in circulators/isolators is achieved by utilizing the “gyromagnetic effect” that is characteristic of ferrite materials. The atoms of these materials are known to have an intrinsic angular momentum (“spin”) and a permanent magnetic moment. When the atoms are exposed to an external biasing magnetic field, a torque normal to the intrinsic angular momentum is applied. The torque causes the magnetic moment of the atoms to “precess” around the magnetic field. Precession refers to a movement of the magnetic moment around the magnetic field lines. From an intuitive standpoint, one may visualize each atom as a spinning top that wobbles on a flat surface, with its individual axis of rotation (e.g., magnetic moment) moving around a fixed vertical axis in a circular motion. When the precession frequency is close to the frequency of the RF signal, an applied magnetic field may be employed to control the propagation of RF signal. In other words, RF signal circulation may be implemented by applying a predetermined DC magnetic field to an appropriately designed ferrite material.
When an RF signal is directed into the input port of the circulator, circulating phase shifted versions of the RF signal are induced within the ferrite discs. The degree of phase shift between counter circulating fields is a function of the strength of the DC magnetic field and diameter of the ferrite material. The circulator operates in accordance with the principles of superposition and constructive/destructive interference of counter-rotating RF waves. Using the example from above, when an RF signal is directed into the first port, the counter circulating RF signals are substantially in phase with each other at the second port, and therefore, they constructively interfere and reinforce each other. The amount of signal available at the second port is measured by what is commonly referred to as the insertion loss. In a properly functioning device the insertion loss is typically in the range of a few tenths of a decibel (dB). At the third port, the RF signals are out of phase with each other and substantially cancel each other. The term “substantially” refers to the fact that, in practice, the cancellation is not perfect and a residual signal may be detected. The amount of residual signal available at the third port, appropriately referred to as the “isolation,” is measured by the ratio of the residual signal and the incident signal. The isolation is typically between −25 dB and −30 dB.
A circulator may be configured as an isolator by terminating one of the ports with a “matched load.” In implementing a matched load, RF engineers ensure that, from an impedance standpoint, the complex impedance of the load is the complex conjugate of the output port impedance. As noted above, an isolator permits RF signal propagation between the two remaining ports in one direction only. RF power flow in the opposite direction is substantially inhibited. Now that the general operating principles have been briefly touched upon, a similarly brief description of the structure of a junction circulator is provided.
A junction circulator includes both electrical and magnetic circuit components and may be implemented using either a stripline or microstrip transmission configuration. The first sub-assembly discussed herein is referred to as the central stack assembly. The electrical portion of the central stack includes a flat center conductor that has three branches extending symmetrically outward from the central conductive portion. The three branches function as the ports of the circulator and are positioned 120° apart from each other. The center conductor is sandwiched between a pair of ferrite discs. The outer surface of both the top ferrite disc and bottom ferrite disc are in contact with ground planes to thereby form a stripline configuration. A permanent magnet is disposed over each ground plane. The permanent magnets apply a predetermined magnetic field to bias the ferrite discs in a predictable manner. A steel pole member may be inserted between each ground plane/magnet pair. The function of the steel pole member is to ensure that the biasing magnetic field applied to the ferrites is substantially uniform. The magnetic properties of both the ferrite material and the magnet may result in temperature variations. Therefore, the central stack may also include thermal compensators that are configured to ensure that the thermal stability of the circulator is maintained. The thermal compensators, which may be fabricated using nickel alloys, offset the aforementioned temperature variations.
In one approach that has been considered, after the central stack is assembled, it is disposed in a housing and secured in place with an interlocking cover plate. The housing and the interlocking cover must apply a certain amount of compression force to the stack to properly secure it within the housing. In a three-port device, the housing may be fabricated having three openings formed in the side walls thereof The openings are configured to accommodate the three ports that extend outwardly from the central conductor. Each port passes through a corresponding one of the three openings and is, therefore, accessible from the exterior of the housing after the assembly of the circulator is completed. Because the housing and the cover compose a part of the magnetic return path, they are typically fabricated using a ferrous metal (e.g., steel) and should have sufficient contact area to transfer the magnetic flux generated by the magnet. From a mechanical perspective, the housing and cover plate must have sufficient mechanical strength to protect the circulator structure from the various mechanical and vibrational forces that may be applied to the structure during its operational life.
The locking arrangement may be realized by forming threads in the inner surface of the housing walls. A second set of threads may be formed around the circumference of the cover plate. The second set of threads formed in the cover plate is, of course, configured to engage the first set of threads formed in the walls of the housing. Once the threads are engaged, a rotational force is applied to the cover plate. The threaded arrangement forces the cover plate downwardly within the housing to thereby apply a compressive force to the stack disposed therein.
Unfortunately, this approach has various drawbacks associated with it. Namely, the manufacturing of the housing and cover is a laborious and expensive process. The process requires several production steps that are performed using turning and milling machines. These production steps are relatively expensive and, therefore, undesirable in a large scale production.
In a second approach, a microwave surface mount circulator having a modified housing arrangement is considered. The circulator under consideration includes a housing fabricated from a single piece of a sheet metal. Six portions are removed from the perimeter of the sheet metal piece to produce a flat piece of sheet metal having six arm structures extending from a central portion thereof The central portion of the sheet metal functions as the bottom of the housing. Subsequently, slanted slots are formed in each of the six arm structures. The six arm structures are then folded up from the bottom portion to form a six-sided polygonal structure. The six side portions are substantially perpendicularly with respect to the bottom portion of the housing and form six flat side walls having slanted slots open at one end thereof.
The second approach includes both a locking cover and a pressing cover. The pressing cover is formed from a piece of ferrous material and has a polygonal shape that matches the geometry of the housing interior. As such, it is configured to fit snugly within the six housing walls under the slanted slots. The locking cover has a circular shape and includes six locking tabs disposed around the perimeter of the plate and extends outwardly therefrom. The locking tabs are configured to mate with the slanted slots disposed in the walls of the housing.
Once the housing and the covers are available, the central stack is disposed within the housing. The pressing cover is disposed within the housing over the central stack. The six locking tabs are inserted into the slanted slots. The locking plate is rotated around the vertical axis of the circulator. The slanted slots force the locking plate to move in a downward direction to apply a compression force to the pressing plate and the central stack. The assembly is essentially complete once the six tabs are interlocked with the slanting slots.
While the second approach under consideration may be deemed an improvement over the first approach considered herein, the locking arrangement described in the second approach has several drawbacks. The polygonal pressing cover, for example, is a necessary component in the second approach under consideration. It is required to prevent any shifting and misalignment of the stack members caused by the rotation of the locking cover. Unfortunately, the pressing cover represents otherwise unusable space between the central stack and the cover. The same applies to the space between the top of locking plate and the top of the side walls, which is necessary to mechanically strengthen the interlocking slots if sufficient stack compression is to be provided. The unusable space directly translates to a circulator component having a relatively larger over-all height dimension, which is, of course, undesirable.
Another drawback in the second approach under consideration relates to the existence of the air gaps between the housing and the covers. The presence of the slanted slots at the side walls is also undesirable. Both of these design features substantially reduce the cross sectional area of the magnetic return path formed by the housing and cover. Because the available magnetic flux in magnetic loop is proportional to the cross sectional area of the magnetic return path, any reduction of the cross sectional area of the magnetic return path directly translates to a reduction of the available magnetic field strength.
Accordingly, it would be desirable to eliminate the locking arrangement and provide an efficient means for enclosing the central stack within the circulator/isolator without requiring any rotational action. What is also needed is a circulator that eliminates the need for a pressing cover. What is also needed is a circulator that substantially reduces the loss of DC magnetic flux by increasing the cross sectional area of the magnetic return path.